Amperometric measurements of cocaine cue and novel context-evoked glutamate and nitric oxide release in the nucleus accumbens core

Further proof on the role of accumbal nNOS in cocaine-seeking behavior in rats

BACKGROUND

Cocaine use disorder (CUD) remains a severe health problem with no effective pharmacological therapy. One of the potential pharmacological strategies for CUD pharmacotherapy includes manipulations of the brain glutamatergic (Glu) system which is particularly involved in drug withdrawal and relapse. Previous research indicated a pivotal role of ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic receptors' type 5 (mGlu5) receptors in controlling the reinstatement of cocaine. Stimulation of the above molecules results in the activation of the downstream signaling targets such as neuronal nitric oxide synthase (nNOS) and the release of nitric oxide.

METHODS

In this paper, we investigated the molecular changes in nNOS in the prefrontal cortex and nucleus accumbens following 3 and 10 days of cocaine abstinence as well as the effectiveness of nNOS blockade with the selective enzyme inhibitor N-ω-propyl-L-arginine hydrochloride (L-NPA) on cocaine seeking in male rats. The effect of L-NPA on locomotor activity in drug-naïve animals was investigated.

RESULTS

Ten-day (but not 3-day) cocaine abstinence from cocaine self-administration increased nNOS gene and protein expression in the nucleus accumbens, but not in the prefrontal cortex. L-NPA (0.5-5 mg/kg) administered peripherally did not change locomotor activity but attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue.

CONCLUSIONS

Our findings support accumbal nNOS as an important molecular player for cocaine seeking while its inhibitors could be considered as anti-cocaine pharmacological tools in male rats.

Chronic hypothalamic-pituitary-adrenal axis disruption alters glutamate homeostasis and neural responses to stress in male C57Bl6/N mice

It is now well-established that stress elicits brain- and body-wide changes in physiology and has significant impacts on many aspects of health. The hypothalamic-pituitary-adrenal (HPA) axis is the major neuroendocrine system mediating the integrated response to stress. Appropriate engagement and termination of HPA activity enhances survival and optimizes physiological and behavioral responses to stress, while dysfunction of this system is linked to negative health outcomes such as depression, anxiety, and post-traumatic stress disorder. Glutamate signaling plays a large role in the transmission of stress-related information throughout the brain. Furthermore, aberrant glutamate signaling has negative consequences for neural plasticity and synaptic function and is linked to stress-related pathology. However, the connection between HPA dysfunction and glutamate signaling is not fully understood. We tested how HPA axis dysfunction (using low dose chronic corticosterone in the drinking water) affects glutamate homeostasis and neural responses under baseline and acute stress in male C57BL/6N mice. Using laser microdissection and transcriptomic analyses, we show that chronic disruption of the HPA axis alters the expression of genes related to glutamate signaling in the medial prefrontal cortex (mPFC), hippocampus, and amygdala. While neural responses to stress (as measured by FOS) in the hippocampus and amygdala were not affected in our model of HPA dysfunction, we observed an exaggerated response to stress in the mPFC. To further probe this we undertook in vivo biosensor measurements of the dynamics of extracellular glutamate responses to stress in the mPFC in real-time, and found glutamate dynamics in the mPFC were significantly altered by chronic HPA dysfunction. Together, these findings support the hypothesis that chronic HPA axis dysfunction alters glutamatergic signaling in regions known to regulate emotional behavior, providing more evidence linking HPA dysfunction and stress vulnerability.

A Subset of Nucleus Accumbens Neurons Receiving Dense and Functional Prelimbic Cortical Input Are Required for Cocaine Seeking

Background

Prelimbic cortical projections to the nucleus accumbens core are critical for cue-induced cocaine seeking, but the identity of the accumbens neuron(s) targeted by this projection, and the transient neuroadaptations contributing to relapse within these cells, remain unknown.

Methods

Male Sprague-Dawley rats underwent cocaine or sucrose self-administration, extinction, and cue-induced reinstatement. Pathway-specific chemogenetics, patch-clamp electrophysiology, in vivo electrochemistry, and high-resolution confocal microscopy were used to identify and characterize a small population of nucleus accumbens core neurons that receive dense prelimbic cortical input to determine their role in regulating cue-induced cocaine and natural reward seeking.

Results

Chemogenetic inhibition of prelimbic cortical projections to the nucleus accumbens core suppressed cue-induced cocaine relapse and normalized real-time cue-evoked increases in accumbens glutamate release to that of sucrose seeking animals. Furthermore, chemogenetic inhibition of the population of nucleus accumbens core neurons receiving the densest prelimbic cortical input suppressed cocaine, but not sucrose seeking. These neurons also underwent morphological plasticity during the peak of cocaine seeking in the form of dendritic spine expansion and increased ensheathment by astroglial processes at large spines.

Conclusion

We identified and characterized a unique subpopulation of nucleus accumbens neurons that receive dense prelimbic cortical input. The functional specificity of this subpopulation is underscored by their ability to mediate cue-induced cocaine relapse, but not sucrose seeking. This subset of cells represents a novel target for addiction therapeutics revealed by anterograde targeting to interrogate functional circuits imbedded within a known network.

Interactions of neuroimmune signaling and glutamate plasticity in addiction

Chronic use of drugs of abuse affects neuroimmune signaling; however, there are still many open questions regarding the interactions between neuroimmune mechanisms and substance use disorders (SUDs). Further, chronic use of drugs of abuse can induce glutamatergic changes in the brain, but the relationship between the glutamate system and neuroimmune signaling in addiction is not well understood. Therefore, the purpose of this review is to bring into focus the role of neuroimmune signaling and its interactions with the glutamate system following chronic drug use, and how this may guide pharmacotherapeutic treatment strategies for SUDs. In this review, we first describe neuroimmune mechanisms that may be linked to aberrant glutamate signaling in addiction. We focus specifically on the nuclear factor-kappa B (NF-κB) pathway, a potentially important neuroimmune mechanism that may be a key player in driving drug-seeking behavior. We highlight the importance of astroglial-microglial crosstalk, and how this interacts with known glutamatergic dysregulations in addiction. Then, we describe the importance of studying non-neuronal cells with unprecedented precision because understanding structure-function relationships in these cells is critical in understanding their role in addiction neurobiology. Here we propose a working model of neuroimmune-glutamate interactions that underlie drug use motivation, which we argue may aid strategies for small molecule drug development to treat substance use disorders. Together, the synthesis of this review shows that interactions between glutamate and neuroimmune signaling may play an important and understudied role in addiction processes and may be critical in developing more efficacious pharmacotherapies to treat SUDs.

Effects of chronic ethanol consumption on the expression of GLT-1 and neuroplasticity-related proteins in the nucleus accumbens of alcohol-preferring rats

Chronic ethanol exposure induces impairments in CNS excitatory and inhibitory activity. These impairments are associated with glutamatergic dysfunction, including altered neuroplasticity. This study examined the effects of 6-week ethanol (15% and 30% v/v) consumption, by male alcohol-preferring P rats, on protein expression associated with neuroplasticity and glutamate transporter-1 (GLT-1) function. The latter regulates intra- and extra-synaptic glutamate levels. We focused on the shell and core subregions of the nucleus accumbens (Acb); i.e., shell (AcbSh) and core (AcbCo), for these measures. Chronic ethanol exposure increased the expression of BDNF, Arc and phosphorylated (p)-post-synaptic density protein-95 (p-PSD-95) in the AcbSh of P rats. Moreover, the ratio of phospho-neuronal nitric oxide synthase (p-nNOS) to total nNOS was also increased in the AcbSh. These changes in BDNF, Arc and p-nNOS/nNOS ratio were not observed in the AcbCo. Furthermore, chronic ethanol consumption reduced GLT-1 expression in the AcbSh. Alternatively, treatment with ceftriaxone (CEF), a known GLT-1 upregulator, abolished the effect of chronic ethanol consumption on BDNF expression in the AcbSh. Overall, the present findings confirm that chronic ethanol consumption modulates activity-associated synaptic proteins, including BDNF, Arc and nNOS in a subregion-specific (i.e., in the AcbSh but not AcbCo) manner. Thus, alterations in mesocorticolimbic glutamatergic homeostasis and neuroplasticity are possible functional targets for the treatment of alcohol use disorders.

A Rat Model of Cocaine-Alcohol Polysubstance Use Reveals Altered Cocaine Seeking and Glutamate Levels in the Nucleus Accumbens

Preclinical models of cocaine use disorder are widely utilized to identify neuroadaptations underlying cocaine seeking and to screen medications to reduce seeking. However, while the majority of cocaine users engage in poly-substance use (PSU), a minority of preclinical studies employ PSU models. We previously reported that when rats consume alcohol after daily intravenous cocaine self-administration, nucleus accumbens (NA) core basal glutamate levels are reduced below those of rats that consumed only cocaine, and do not increase during cue + cocaine-primed reinstatement of cocaine-seeking. Here we used the same model of sequential cocaine and alcohol self-administration to test the hypothesis that a similar pattern of glutamate changes would be observed in the NA core prior to and during a cocaine-primed reinstatement test. Rats underwent intravenous cocaine self-administration followed by access to unsweetened alcohol in the home cage for 12 days. Rats underwent a minimum of 12 daily extinction sessions prior to a cocaine-primed reinstatement test conducted during microdialysis procedures. Contrary to our previous work using the same model, here we found that access to alcohol increased cocaine intake and increased responding during early extinction training. We found that as in our previous work, cocaine + alcohol-consuming rats displayed basal glutamate levels below those of rats that self-administered only cocaine. During the cocaine-primed reinstatement test, rats that consumed only cocaine displayed increased glutamate efflux in the NA core while those that consumed cocaine + alcohol did not. These results indicate that preclinical models of PSU should be utilized to develop experimental therapeutics for the reduction of cocaine seeking.